Acid has little effect on periostracum, but has an extreme effect on
carbonate salts, which is what the shell is made of. Muriatic acid won't
remove the periostracum from the shell, but it will quickly remove the shell
from the periostracum. Vinegar and other weak acids will do the same thing,
just more slowly. Organic solvents like alcohols, acetone, hydrocarbons,
etc. will have no effect either on shell or periostracum. You definitely
need a fairly strong base to dissolve periostracum without dissolving shell.
I am very surprised that bleach didn't work. By "bleach" I mean a liquid
bleach containing hypochlorite - like Chlorox. That should dissolve
periostracum. If it doesn't, you can try oven cleaner or drain cleaner, or a
solution of sodium hydroxide or potassium hydroxide (I recommend these in
view of your stated use for the shells. I wouldn't recommend these methods
for specimen shells to be placed in a collection). Handle these chemicals
with care. They also dissolve skin.

Paul M.